The field of the invention relates to high temperature superconducting (HTS) radio frequency (RF) filters for wireless communications. Specifically, the invention relates to a bypass system used in connection with a HTS-based filter and a cryogenically cooled low noise amplifier (LNA).
The increase in the number of mobile telecommunication devices in recent years and the corresponding increase in the amount of data capacity required for such devices has led to the development and implementation of HTS-based RF filters used in connection with RF front-end devices. HTS filters are highly selective, low loss filters that substantially decrease interference between adjacent channels. One of the benefits of the decrease in interference between adjacent channels is the reduction of the number of calls that are dropped. This is particularly advantageous when an ever increasing number of users are occupying frequencies that are very close together. HTS-based filters also provide better mobile-to-base call quality due to the filter""s increased sensitivity. Another important benefit of HTS-based front-end RF filters is that wireless providers can increase the capacity (i.e., call capacity) of existing, non HTS-based base stations. Wireless providers can also deploy wireless networks having fewer base stations when such base stations include HTS-based filters.
HTS front-end RF filters require the use of a cryocooler to cool the filters and any associated electronics, such as LNAs, to around 77K. Accordingly, it is preferable that the cryocooler used to cool the filters be able to operate for long periods of time and in a variety of environmental conditions without failing. Stirling cycle cryocoolers, for example, have been developed and used to cool HTS components for extended periods of time without interruption. In some applications, however, a bypass feature is needed in case one or more of the HTS filters contained in the RF front-end fails to perform properly, as would be the case if the cryocooler failed. It is known, for example, to include a bypass feature on a cryogenically cooled receiver front-end that uses two conventional RF relays to bypass the HTS circuitry to direct the antenna signal directly to an output that proceeds to the base station.
Bypass systems that use conventional RF relays, however, have a number of limitations. First, the conventional RF relays have a relatively high insertion loss, which degrades the overall noise figure of the RF receiver. Consequently, when the system is in bypass mode, reverse channel (i.e., mobile-to-base) coverage is reduced. Second, conventional RF relays dissipate power even in their quiescent state when operated in a fail-safe mode (i.e., if there is a power failure, the relays default to the bypass mode). Consequently, conventional RF relays must be located external to the cryogenic enclosure to avoid the relays using up the finite thermal budget of the cryocooler. Third, conventional RF relays are rather large devices that increase the overall size and weight of the RF receiver.
Accordingly, there is a need for a bypass system for an HTS-based filter/LNA RF front-end receiver that has a very low insertion loss, i.e., a very low contribution to the noise figure of the overall receiver. There also is a need for a bypass system in which the individual switching elements can be located inside the cryogenic enclosure. Accordingly, the individual switching elements need to have little or no power dissipation when switches are in their quiescent state.
In a first aspect of the invention, an HTS-based RF receiver includes a cryocooler, a cryogenic enclosure in thermal communication with the cryocooler, a RF input, and a RF output. The receiver also includes a HTS filter having an input and an output, the input of the HTS filter being operatively coupled to the RF input, the output of the HTS being coupled with a LNA, the LNA having an output that is operatively coupled the RF output. The HTS filter and the LNA are disposed within the cryogenic enclosure. A first Micro-Electro-Mechanical System (MEMS) bypass switch is provided between the RF input and the HTS filter, the first MEMS bypass switch operatively coupling the RF input to the HTS filter. A second MEMS bypass switch is provided between the LNA and the RF output, the second MEMS bypass switch operatively couples the LNA to the RF output. A bypass pathway located in the cold space of the cryogenic enclosure is connected between the first and second MEMS bypass switches.
In a second, separate embodiment, the HTS RF receiver of the first embodiment is modified such that the output of the HTS filter is operatively coupled with the LNA. The second MEMS bypass switch is positioned between the HTS filter and the LNA to operatively couple the HTS filter and LNA.
In another embodiment of the-invention, a method of bypassing a HTS filter in a RF receiver containing a HTS filter and a LNA includes the steps of measuring an operating parameter of the RF receiver, and switching the RF receiver to a bypass mode when the measured operating parameter is outside a pre-determined operating range, the step of switching the RF receiver to the bypass mode includes the step of switching two MEMS switches to a bypass pathway around the HTS filter.
In still another embodiment, the method of bypassing described above includes the step of switching two MEMS switches to a bypass pathway around the HTS filter and the LNA.
It is an object of the invention to provide a HTS-based RF receiver with a bypass capability. It is another object of the invention to provide a MEMS-based bypass solution that reduces the overall size of the device and permits the bypass switches to be placed inside a cryogenic environment. It is yet another object of the invention to provide a low insertion loss bypass system that is used with HTS-based RF receivers.